1. Field of the Invention
The present invention relates to a method for bonding two surfaces to one another. The invention particularly pertains to the use of such method in which one of the surfaces is a polymeric plastic (and more preferably a polymeric thermoplastic (especially poly-(methyl methacrylate) (“PMMA”) or cyclic olefin copolymer (“COC”)). More particularly, the invention relates to treating at least one of the contacting surfaces with UV in the presence of oxygen to thereby generate ozone (O3) and atomic oxygen under conditions of temperature below that of the glass transition temperature of the polymeric plastic. The UV/O3-mediated bonding results in high bond strength and zero-deformation method. This bonding method can be applied to micro/nano-scale polymer devices, and particularly to microfluidic devices, for a low cost, high throughput, high yield advantage.
2. Description of Related Art
Rigid thermoplastic polymers have been extensively investigated over the past decade as substrates for the fabrication of microfluidic systems. Two particular polymeric thermoplastics: poly(methyl methacrylate) (“PMMA”) and cyclic olefin copolymer (“COC”), have emerged as attractive materials for microfluidic applications, primarily due to their high transparency and low autofluorescence over a wide spectral range (Piruska, A. et al. (2005) “The Autofluorescence Of Plastic Materials And Chips Measured Under Laser Irradiation,” Lab on a Chip 5:1348-1354). In a typical process flow, open microchannels are formed in first a thermoplastic substrate using one of several techniques such as hot embossing (Martynova, L. et al. (1997) “Fabrication of Plastic Microfluid Channels by Imprinting Methods,” Anal. Chem. 69:4783-4789), cold embossing (Xu, J. et al. (2000) “Room-Temperature Imprinting Method for Plastic Microchannel Fabrication,” Anal. Chem. 72:1930-1933), micro-injection molding (McCormick, R. M. et al. (1997) “Microchannel Electrophoretic Separations of DNA in Injection-Molded Plastic Substrates,” Anal. Chem. 69:2626-2630), or laser ablation (Roberts, M. A. et al. (1997) “UV Laser Machined Polymer Substrates for the Development of Microdiagnostic Systems,” Anal. Chem. 69:2035-2042). A second plastic layer is then bonded to the first to enclose the microchannels.
A variety of bonding methods have been reported, including solvent bonding (Kricka, L. J. et al. (2002) “Fabrication Of Plastic Microchips By Hot Embossing,” Lab on a Chip 2:1-4; Klank, H. et al. (2002) “CO2-Laser Micromachining And Back-End Processing For Rapid Production Of PMMA-Based Microfluidic Systems,” Lab on a Chip 2:242-246; Brown, L. et al. (2006) “Fabrication And Characterization Of Poly(Methylmethacrylate) Microfluidic Devices Bonded Using Surface Modifications And Solvents,” Lab on a Chip 6:66-73), thermal bonding (Sun, Y. et al. (2006) “Low-Pressure, High-Temperature Thermal Bonding Of Polymeric Microfluidic Devices And Their Applications For Electrophoretic Separation,” J. Micromech. Microeng. 16:1681-1688; Kelly, R. T. et al. (2003) “Thermal Bonding of Polymeric Capillary Electrophoresis Microdevices in Water,” Anal. Chem. 75:1941-1945), and thick film lamination employing either pressure or temperature sensitive adhesive layers (do Lago, C. L. et al. (2003) “A Dry Process for Production of Microfluidic Devices Based on the Lamination of Laser-Printed Polyester Films,” Anal. Chem. 75:3853-3858).
Of these techniques solvent and thermal bonding are of particular interest in the fabrication of microfluidic devices since they allow the same material to be used for both microfluidic substrate layers, and thereby ensure homogeneity in surface properties for all microchannel walls. In thermal bonding, interlayer adhesion is achieved by beating the substrates near their glass transition temperature while applying a normal pressure, allowing polymer chains to diffuse between the mating surfaces for high bond strength. However, thermal bonding suffers from several disadvantages. Because the substrates must be heated at or slightly above their glass transition temperature to achieve a strong interfacial bond, microscale channels can readily become deformed or collapsed, particularly for low aspect ratio channels and thin substrates. Furthermore, the resulting bond strength is often lower than desired, particularly for applications such as liquid chromatography where high internal fluid pressures are required. Solvent bonding can also suffer from problems with dimensional stability, since the absorbed solvent softens the plastic and can lead to polymer flow during bonding. While recipes have been developed to minimize this problem in PMMA microfluidic chips by using specific solvent conditions (see, Brown, L. et al. (2006) “Fabrication And Characterization Of Poly(Methylmethacrylate) Microfluidic Devices Bonded Using Surface Modifications And Solvents,” Lab on a Chip 6:66-73; Lin, C. H. et al. (2005) “Low Azeotropic Solvent Sealing Of PMMA Microfluidic Devices,” Proc. 13th Int. Conf. Solid-State Sensors, Actuators, and Microsystems (Transducers 05) 1:944-947), or sacrificial materials such as paraffin wax to prevent channel collapse (see, Kelly, R. T. et al. (2005) “Phase-Changing Sacrificial Materials for Solvent Bonding of High-Performance Polymeric Capillary Electrophoresis Microchips,” Anal. Chem. 77:3536-3541), the former recipes must be tuned for different polymer grades and types, and neither approach can entirely prevent deformation of channel geometries. Furthermore, solvents can embrittle thermoplastics and result in microcracking, particularly for microfluidic systems which require exposure to high or cyclical pressure loads.
Because of these challenges, there remains a need for effective methods for low temperature thermoplastic bonding, and which, in particular, are amenable to a wide range of microfluidic applications. The present invention is directed to this and other needs.